In recent years, a digital camera has been reduced in size and a portable terminal is provided with a digital camera. A user can easily take photos of landscapes, documents or characters/drawings displayed on a whiteboard by a portable terminal with camera even when the user is away from home. However, while the user can easily take photos even when the user is away from home, a shooting location may be limited due to a condition of the user's standing position, a blackboard may be difficult to photograph from its front due to a position of the user's seat, or a shooting angle may be limited due to a relationship between a target to be photographed and an angle of light when, for example, the user goes hiking in the mountains or finds a desired target.
In order to correct an image of a target to be photographed such as documents or characters written on a whiteboard and to obtain an image which seems to be photographed from its front, for example, Patent Document 1 discloses therein an image processing apparatus for obtaining a contour by use of a Roberts filter from a photographed image of a target whiteboard, detecting candidate straight lines forming the image of the whiteboard from the obtained contour, obtaining a quadrangular shape of the whiteboard, and finding projective parameters indicating the relationship between the image of the whiteboard and the actual whiteboard based on the vertexes of the quadrangle to projectively transform the image of the whiteboard.